1. Field of the Invention
The present invention relates generally to valve actuators and, more particularly, to apparatus and methods for pneumatically operated surface gate valve actuators.
2. Description of the Background
Pneumatic actuators are often used as a fail-safe device that responds to a low pressure in a gas supply, such as might occur due to a rupture or other failure whereupon the actuator returns the gate valve to a safe position. Conventional pneumatic actuators often have numerous problems that increase the cost of manufacturing, decrease the reliability, and make maintenance more time consuming and difficult. In many devices, it is difficult for workmen to align the various components during assembly so that considerable time is required for both assembly and disassembly. Since pneumatic actuators are relatively large and heavy and may range from about two feet to about four feet in length and weigh upwards of a thousand pounds or more, assembly and disassembly time problems can be significant cost factors. In many prior art devices, the actuator and gate valve design are designed as a single unit so that it is necessary to disassemble or assemble major portions of both actuator and gate valve for any maintenance as well as for manufacturing assembly purposes. In some cases, it is necessary to preload during assembly the return spring within the pneumatic actuator that provides the failsafe operation. This increases assembly time and also increases the time and equipment needed for disassembly. This construction can cause manufacturing delays and/or can greatly increase the time necessary for replacing, for instance, a single seal. The stem seals and/or piston seals can be very difficult to replace in some designs and may require replacing many additional seals that are functioning effectively, including metallic ring seals that are preferably left undisturbed when functioning properly. Often different size actuators require completely different sets of parts so that cost reductions due to component interchangeableness is lost. It is sometimes difficult to even match the various gas supply pressures typically available for operation to the type of actuators with the desired stroke for the pipe bore size.
Some previous attempts to overcome problems of pneumatic actuators such as the above and others are shown in the following representative patents for pneumatic actuators.
U.S. Pat. No. 4,135,546, issued Jan. 23, 1979, to B. L. Morrison, discloses an actuator with a packing assembly arranged between the gate valve stem and a bonnet secured to the valve body. An externally threaded packing sleeve is threaded within the bonnet and has the packing assembly removably connected thereto for movement with the sleeve.
U.S. Pat. No. 3,993,284, issued Nov. 23, 1977, to F. W. Lukens, Jr., discloses a mount for securing an actuator cylinder on a gate valve wherein the valve has a stem extending through a sleeve on the valve bonnet and the actuator cylinder has a collar mounted on the sleeve. A split ring is mounted in a groove in the outer end portion of the sleeve at the collar. A means mounted on the sleeve extends over the split ring to detain it in the mounted position.
U.S. Pat. No. 4,871,143, issued Oct. 3, 1989, to G. S. Baker, discloses a gate valve with a supplemental actuator having a connection to the actuator housing to exert an additional closing force on the stem.
U.S. Pat. Reissue No. 29,322, reissued Jul. 26, 1977, to N. A. Nelson, discloses a mechanism for remote actuation by a pressure medium such as hydraulic or pneumatic fluid. Mechanical or fluid actuated interlock means is provided to render fluid actuating means inoperative and is also provided with means for rendering the mechanical override inoperative.
A review of the above patents shows that there remains a need for a pneumatic actuator assembly that offers more dependable operation, faster maintenance, easier assembly and disassembly, all at reduced levels of capital investment. Those skilled in the art have long sought and will appreciate the present invention which addresses these and other problems.